The present invention relates generally to high temperature electric heaters for use as nuclear fuel rod simulators in testing nuclear reactors, and more particularly to the fabrication of an improved fuel rod simulator which closely resembles an actual nuclear fuel rod or pin.
The present invention is directed to an improved high temperature electric heater particularly useful as a nuclear fuel rod simulator in safety and thermal hydraulic studies in nuclear ractors. Nuclear fuel rod simulators of the type described herein use electrical energy to simulate the heating produced in a stack of nuclear fuel pellets and have been previously utilized in reactor studies. For example, one such nuclear fuel rod simulator is described in commonly assigned U.S. Pat. No. 4,106,186 issued to Chester S. Morgan, Jr. et al, Aug. 15, 1978 and entitled "Boron Nitride Insulating Material." Inasmuch as the nuclear fuel rod simulator of the present invention represents a modification of the fuel rod simulator described in the aforementioned patent, the teachings of this patent are incorporated herein by reference.
As described in the aforementioned patent, the high temperature fuel rod simulators each comprises a cartridge-type assembly in which a cylindrical metal housing is provided with a longitudinally and concentrically disposed heating element with this heating element being separated from the metal housing by boron nitride (BN). The boron nitride provides both electrical insulation and sufficient thermal conductivity to provide the simulator with a moderately high heat flux in the range of about 100 to 400 watts/cm.sup.2. The electric heating element disposed within the metal housing is a helically wound ribbon of Nichrome V or Kanthal A-1 through which an electric current is passed. The cavity within the electric heating element formed by the helical winding is similarly filled with boron nitride insulating material.
Prior to the development in cartridge-type heaters as described in assignee's aforementioned patent, the annulus between the heating element and the shell, or housing, and the cavity in the heating element were filled with boron nitride powder. The tamping of this "loose" powder was attempted by compacting the powder with tools inserted into the annulus and the heating element cavity. Inasmuch as this powder composition required the use of tamping tools of dimensions closely approximating the dimensions of the annulus and the heating element cavity, considerable abrasion and displacement of metal from the heating element, tool, and the shell occurred which significantly reduced the voltage breakdown potential and electrical resistance of the heater and often led to heater malfunctions due to the presence of this displaced metal in the insulating material. Further, the tamping of the "loose" boron nitride powder resulted in initial density levels of 50 to 63% theoretical and thus required a great deal of swaging of the fuel rod simulator to provide the desired density of about 95% theoretical of the insulator. This often resulted in excessive deformation and failure of internal thermocouples, heating elements, and other heater components.
Efforts to provide insulators of increased density to decrease the amount of swaging required included the use of extruded and sintered cylinders of magnesia (MgO) in the heating element cavity. The use of these magnesia cylinders did not prove to be satisfactory since the transient profile uniformity requirements of .+-.5% in a 40.degree. C./sec transient could not be attained because of the deleterious effects of variations in density (heat capacity) of the MgO. Additionally, hardness variations in the magnesia resulted in diameter variations in the heating element which in turn caused excessive heat flux variations.
Several other problems were also introduced by swaging, even if done in smaller amounts. For example, internally confined thermocouples were frequently sufficiently damaged so as to significantly decrease useful heater lifetime. Also, tolerances in the dimensions of the cartridge heater were increased by swaging so as to cause substantial variations from the dimensions desired.
Another disadvantage associated with the swaging of the assembled heaters is due to the orientation of the boron nitride particulates forming the insulator. It has been found that the orientation of the basal planes of the boron nitride particulates in the radial direction with respect to the longitudinal axis of the heater provides significantly greater thermal conductivity while maintaining desired electrical insulating properties than the orientation of the basal planes of the boron nitride particulates along the longitudinal axis of the heater. Swaging causes the basal planes of the boron nitride particulates to be oriented substantially along the longitudinal axis of the heater.
The fabrication technique utilized to fabricate cartridge-type heaters with boron nitride insulators as described in assignees' aforementioned patent overcomes many problems encountered in the fabrication of heaters previously used for nuclear fuel rod simulations. As pointed out in this patent, the annular region between the heating element and the housing and the cavity within the heating element are both provided with boron nitride insulation. The boron nitride powder was cold pressed into preforms having a length-to-diameter ratio of about 3:1. These preforms were then inserted into the cavity in the heating element and then in the annular region between the heating element and the housing. The preforms were preferably inserted into the cavity and the annulus one at a time and individually crushed by a tool of dimensions insufficient to damage the housing, or the heating element as previously encountered. By utilizing the cold-pressed preforms the boron nitride is at a considerably higher and more uniform density than previously provided by tamping loose powder and the thermal conductivity through the insulation is more uniform than achievable by the significantly deformed or swaged heaters as previously utilized. This allows for much less swaging of the heater to reach the desired levels of density and density uniformity.
While the fabrication techniques disclosed in assignees' aforementioned patent provided cartridge heaters with significantly improved operating characteristics over those previously obtainable, there were still several shortcomings which detracted from the overall desirable features of the heater. For example, even though the use of the crushable preforms significantly reduced the extent of swaging required, the swaging of the preforms with a diameter reduction of about 1 to 3% was found to be necessary in order to achieve the required final density and thermal contact between the boron nitride and the housing.
It was also found that without utilizing a swaging operation that the differences in the coefficient of thermal expansion between the boron nitride insulator in the annulus and the sheath or housing caused the housing to be radially displaced away from the boron nitride at elevated temperatures so as to significantly reduce the contact between the insulator and the housing and thereby substantially reduce the thermal conductivity through the insulator. Further, even with the slight extent of swaging utilized in the heater fabrication described in the aforementioned patent, the choice of the sheath material was necessarily limited to metals capable of undergoing the cold working encountered during the swaging operation. Therefore, materials normally used as nuclear fuel cladding such as zircaloy, "roughened" or ribbed stainless steel, or uniformly prestained stainless steel which are virtually nonswagable, could not be used as the housing material for the heaters so that simulation of the actual nuclear fuel pin assemblies could not be as accurate as achievable with heaters using actual cladding material. Also, even with a relatively small degree of swaging such as utilized in the aforementioned patent, the thermo-couples disposed within the annulus between the heating element and the housing were prestrained to an extent so as to reduce their useful lifetime to one fifth that normally encountered with no prestrain.